A number of techniques are known for characterising the properties of formations surrounding boreholes. Typically, a tool body comprising a signal source is placed in the borehole and the source used to interrogate the formation. A signal returning to the tool body is measured and the parameter of interest calculated. Where the tool body is positioned tight against the borehole wall, the signal essentially only passes through the formation surrounding the borehole and so can be used in a straightforward manner to derive the property of interest. However, where there is a space between the tool body and the borehole wall (sometime called ‘standoff’), problems can arise because the signal passes through the contents of the borehole before passing into the formation or returning from the formation to the tool body. Thus, the contents of the borehole, typically fluids, will have an effect on the signal and this effect must be removed, or compensated, to determine the formation property of interest.
One well-known way to compensate for borehole effects is to make two measurements of the returning signal at differing distances from the source. Over the scale of a typical measurement, the standoff and borehole effects do not vary substantially between the near and far measurement locations, such that the only significant difference between the two measurements is due to the difference in the signal path in the formation, and hence the formation property of interest.
The measurement of formation density is a well-known way of characterising underground formations surrounding oil and gas wells and the like. There are a number of techniques based on nuclear measurements that can be used to do this. The most common technique is known as gamma-gamma density logging in which the formation is irradiated from a gamma-ray source, e.g. 137Cs emitting gamma rays at 0.66 MeV, in the tool body. The gamma rays pass through the formation, undergoing interactions with the constituents of the formation and return to the borehole where the flux is detected. This can then be analysed to determine formation density. This technique has been used for both wireline and LWD applications for well logging. To compensate for borehole effects, the dual-detector approach described above is frequently used. U.S. Pat. No. 4,297,575 describes a gamma-gamma density logging method.
Because gamma-gamma density logging provides a bulk formation density, and underground formations are typically porous, it has been proposed to combine gamma-gamma density logging with a neutron measurement of porosity. In neutron porosity logging, the formation is irradiated with high-energy neutrons from either a chemical source, e.g. 241AmBe producing 4 MeV neutrons, or an accelerator source, e.g. a pulsed D-T accelerator producing 14 MeV neutrons. As they pass through the formation, they interact with hydrogen nuclei in the pore fluids, scattering and losing energy. The scattered neutrons are detected back in the tool body and can be analysed to determine the amount of hydrogen, and hence the porosity of the formation. Neutron porosity measurements can be combined with gamma-gamma density measurements to allow determination of the density of the rock matrix, as is described in WO2007/149869. Again, dual detector compensation techniques have been adopted to deal with borehole effects.
More recently, techniques for determining formation density based on irradiation with high energy neutrons have been developed. Neutrons will be scattered by the formation depending on its density. Thus, measuring the returning neutrons can be used to obtain some determination of the formation density (neutron-neutron density). However, this can be subject to a number of interfering effects from the porosity effect of the formation. Neutron irradiation can also lead to inelastic scattering of neutrons with accompanying generation of gamma rays. These gamma rays can be used in a density determination method (neutron-gamma density). In this case, the formation is effectively acting as a secondary source of gamma rays from which density can be determined. WO96/08733, U.S. Pat. No. 5,608,215, and US2009/026359 disclose neutron-neutron and neutron-gamma techniques for density determination.
There are two common techniques for analysing dual-spaced nuclear measurements. In one, a simple cross-plot or ratio of the signals measured at the two detector spacings is used and compared to calibrated data from known formations. This approach is commonly used in neutron-neutron porosity and density and neutron-gamma density. Another is known as spine and rib processing and is used on gamma-gamma density. This approach is based on determination of a cross-plot of the measurements at the two detector spacings and the determination of a deviation to this cross plot due to tool standoff, the intersection of the cross-plot (spine) and deviation (rib) being used to determine the formation density Examples of these approaches can be found in the documents cited above.
This invention is based on the recognition that the spine and rib approach can be applied in neutron-gamma density techniques even though the fundamental process underlying the measurement (creation of gamma rays in the formation due to neutron irradiation) is substantially different to that of gamma-gamma density techniques relying on direct irradiation with gamma rays of known energy.